Device and method for multiplexing cells of asynchronous transmission mode

ABSTRACT

The device and method for multiplexing cells inputted to an asynchronous transmission mode using an input buffer according to the amount of traffic. The cell multiplexing device of an asynchronous transmission mode, includes: a buffer portion which include cell buffers corresponding to input ports, for storing cells received through the input ports; counters which have the number corresponding to that of the buffers, for storing the number of cells to be stored in the buffers a unit clock unit for determining for the counters a count unit of cells to be stored in the buffers; selector for outputting control signals to select buffers having their maximum values by comparing the values of the counters with each other; and a unit for inputting the output of the buffer unit and multiplexing the input cells according to the control signal, wherein the generation of buffer overflow is prevented by primarily processing the cells from a buffer where the most cells are stored.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. application Ser.No. 08/774,178, filed in the U.S. Patent & Trademark Office on the 26thday of December 1996 and issued as U.S. Pat. No. 5,982,777 on Nov. 9,1999, U.S. application Ser. No. 08/774,178 being incorporated herein byreference. Also, this application makes reference to, incorporates thesame hereins and claims priority and all benefits accruing under 35U.S.C. §120 from the aforementioned U.S. application Ser. No.08/774,178, and issued as U.S. Pat. No. 5,982,777, entitled Device andMethod for Multiplexing Cells of Asynchronous Transmission Mode.

CLAIM OF PRIORITY

This application makes reference to, incorporates the same herein, andclaims all benefits accruing under 35 U.S.C §119 from an applicationentitled Device and Method for Multiplexing Cells of AchronousTransmission Mode earlier filed in the Korean Industrial Property Officeon the 26th of December 1995, and there duly assigned Ser. No.56582/1995 by that Office.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to multiplexing devices and processesgenerally, and more particularly, to a device and process formultiplexing cells by dynamically controlling a buffer.

2. Description of the Related Art

In general, a multiplexing device sequentially selects and transmitsinput data. A Round Robin multiplexing system is configured tosequentially transmit the data (cell) received in buffers that areprovided at each input port. Typically, a multiplexing devicesequentially reads the data received to each of the buffers. The RoundRobin system reads and transmits the data (cell) stored in acorresponding buffer while making an orderly selection of each of thebuffers. I have found however, with multiplexers using a Round-Robinsystem, that the cell can be lost due to buffer overflow or that bufferdelay can occur when traffic concentrates on a particular buffer.

The traffic concentration at a particular buffer during multiplexing asabove occurs as follows. First concentration of traffic can occur whenthe traffic originates due to burstiness of the source. Second, duringthe internal for the routing process in the network, the concentrationof traffic occurs when a plurality of incoming traffic is received forthe same point of destination. Third, the traffic concentrates when theburstiness increases in the traffic due to jitter occurring due todiverse delays inside the net. Accordingly, in multiplexing devicesusing the Round Robin system, when the input traffic is not uniform andthe burstiness becomes strong, the number of the cells being received ata particular cell will correspondingly increase. Thus, cell loss canoccur due to buffer overflow if attributable to the concentration of thetraffic; also, cell transmission delay can occur inside the buffer. Ihave observed that since the use efficiency of the buffer deterioratessubstantially in the event that the number of the cells received by aparticular buffer is substantially reduced, multiplexing devices usingthe Round Robin system are not effective when the traffic is not uniformand when the destination of the traffic tends to be concentrated.

Earlier efforts in asynchronous transmission mode cell switching, suchas the Supervision Control System of Yoshimura et al., U.S. Pat. No.5,394,396, and the Arrangement For Controlling Shared-buffer-memoryOverflow In A Multi-priority Environment of Pashan, et al., U.S. Pat.No. 5,233,606, depended upon circuits that not infrequently discardedcells. Other efforts such as the Method And A System Of Control OfAsynchronous Time Communication Outputs of Boyer, et al., U.S. Pat. No.5,299,191, depended upon continuous comparisons of buffer occupancy intime division multiplexing.

Designs such as the Output-Buffer Switch For Aschronous Transfer Mode ofKenji Yamada, U.S. Pat. No. 5,455,820, endeavored to avoid loss of cellsby operating a buffer controller on the basis of a determination ofoccupancy ratio relative to a predetermined threshold. These designshowever, seem to require an excessive number of buffers, and incurconcomitant delay in transmission of the multiplexed cells.

More recent efforts in the art, such as the Asynchronous Transfer Mode(ATM) Multiplexing Process Device And Method Of The Broadband IntegratedService Digital Network Subscriber Access Apparatus, by Seung W. Shon,U.S. Pat. No. 5,499,238, for example have sought to use a complexranking of cells using determinations of whether a buffer is full andthe class order of each of the cells. Efforts to address the problemsattendant to non-uniform traffic and undue concentration can not bereliably addressed however, on a classification scheme that depends inpart upon the occurrence of full buffers.

SUMMARY OF THE INVENTION

It is therefore, one object of the present invention to provide animproved multiplexing circuit and process.

It is another object to provide a multiplexing circuit and process ableto accommodate non-uniformly distributed traffic to the input ports ofthe circuit.

It is still another object to provide a multiplexing circuit and processable to accommodate excessive concentration of traffic to the same pointof destination.

It is yet another object to provide a device and process formultiplexing cells received in an asynchronous transmission mode usingan input buffer, in dependence upon the amount of traffic.

It is still yet another object to provide a simple and reliableasynchronous transfer mode multiplexer and process.

Accordingly, to achieve these and other objects, there is provided acell multiplexing device of an asynchronous transmission modeconstructed with a buffer stage including cell buffers corresponding todifferent input ports, for storing cells received through the inputports. Counters corresponding in number to the buffers, store thenumbers of cells to be stored in the buffers; a unit clock unitdetermines a count unit of cells to be stored in the counters; and aselector generates control signals for selecting those buffers holdingtheir maximum capacity of value by comparing the values of the counterswith each other. A multiplexer unit receives the output of the bufferunit and multiplexes the input cells in accordance with the controlsignal. Generation of buffer overflow is prevented by primarilyprocessing the cells read from the buffer where the most cells arestored.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention, and many of the attendantadvantages thereof, will be readily apparent as the same becomes betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings in which likereference symbols indicate same or similar components, wherein:

FIG 1 is an abstract diagram illustrating the configuration of ahypothetical representation of contemporary multiplexing practice in theart using a Round Robin system;

FIG. 2 is a diagram illustrating the occurrence of buffer overflow inthe multiplexing device illustrated by FIG. 1;

FIG. 3 is a diagram illustrating the configuration of a device formultiplexing cells in an asynchronous mode according to the principlesof the present invention; and

FIG. 4 is a flowchart for explaining a method for multiplexing cells inan asynchronous mode according to the principles of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the attached drawings, FIG. 1 shows the configuration ofa prior art multiplexing device using a Round Robin system. Multiplexingdevice 50 sequentially multiplexes the data received at input ports 1, 2. . . n, and buffers bf1 to bfn store the data cells received at eachinput port. With the configuration shown, multiplexing device 50performs multiplexing by sequentially reading the data cells stored ineach buffer bf1-bfn. By using a Round Robin system, multiplexing device50 reads and transmits the data cell stored in a corresponding bufferwhile sequentially selecting each of buffers bf1 to bfn. Whenmultiplexing device 50 uses such a RoundRobin system, and trafficconcentrates on a particular buffer, the cell may be lost due to bufferoverflow or, alternatively, buffer delay can happen, as is representedby FIG. 2.

FIG. 2 is a diagram illustrating the occurrence of buffer overflow inmultiplexing device 50. The traffic concentration on a particular bufferduring multiplexing can vary, with adverse affects upon reliability, asfollows. First, when the traffic itself occurs due to burstiness, thatis, excessive bursts of data from the source, the traffic becomes undulyconcentrated. Second, during the internal routing process of thenetwork, the concentration of traffic occurs when a plurality of trafficis received that is destined for the same point. Third, traffic becomesunduly concentrated when burstiness increases due to jitter occurringfrom diverse delays inside the net. Consequently, with multiplexingdevice 50 using the Round Robin system as above, when the input trafficis not uniform and the burstiness becomes strong, the number of thecells being received at a particular port will be correspondinglyincreased. Thus, cell loss can occur due to the buffer overflow byconcentration of the traffic, and also, cell transmission delay canoccur inside the buffer. Since the use efficiency of the buffer isgreatly reduced in the event that the number of the cells to be storedin a particular buffer is reduced, multiplexing device 50 is notespecially effective when the traffic is either not uniform and or thedestination of the traffic is unduly concentrated.

Services to be provided through a network of an asynchronoustransmission mode has the widespreading range of traffic speed as wellas various traffic features. However, since the resource of the networkhas a limit, a method for managing the network to simultaneously provideall the services without deterioration of quality is necessarilyrequired. The present invention provides a method of performingefficiently using a multiplexing device by controlling a cell bufferaccording to a traffic amount which is preset in a multiplexing devicefor multiplexing cell traffic. Thus, the present invention can furtherimprove service quality factors such as cell damage or cell transmissiondelay capable of occurring when a designated buffer having a fixed sizeis used for every cell input port, and has an effect of sharing a bufferby all ports in terms of efficiency in using the buffer.

FIG. 3 is a diagram illustrating the configuration of a device formultiplexing cells of an asynchronous transfer mode (i.e.,.ATM)according to the principles of the present invention. The multiplexingdevice is constituted with a buffer unit 10, a first controller 20, asecond controller 30 and at third controller 40. First, buffer unit 10can be constructed with a plurality of discrete buffers 1-n, namely BF1to BFn. The buffers BF1 to BFn are for storing and reading cells to beinputted namely, CELL INPUTS 1-n, and each buffer BF is also comprisedof a cell buffer 11, a tri-state buffer 12 for controlling andgenerating a write A clock CLKwr and a counter 13 for controlling andgenerating a read clock CLKrd. The buffers BF1 to BFn are connected tothe input port to which each input port of the buffer BF1 to BFncorresponds by one-to-one correspondence and an output port of thebuffer BF1 to BFn is connected to a multiplexer 42 of the thirdcontroller 40.

The cell buffer 11 stores a cell received being connected to the inputport. The tri-state buffer 12 provides the write clock CLKwr to therebystore the cell inputted to the cell buffer 11 when a write control clockis inputted, and performs a function of controlling the write clock tobe supplied when a valid cell is inputted. The counter 13 outputs theread clock CLKrd to read the cell stored in the cell buffer 11. The readclock is controlled by the counter 13 on the purpose of supplying theread clock CLKrd as much as the number of the cell to be readsequentially at a time from the selected cell buffer 11.

The first controller 20 counts the number of the cells stored in eachbuffer BF1 to BFn and has a function of searching a buffer which storesthe greatest number of cells by comparing the counted cell values witheach other. The first controller 20 is comprised of counters CLKwl toCLKwn, 211 to 21n for counting the CLKwr where cells are actually storedin each buffer BF1 to BFn, a selector 23 for searching the maximum valueby comparing the count values with each other and a unit clock counterCNTi 22 for setting the unit of the counter. The unit clock counter 22also determines how many cells will be counted as a unit when the cellsto be stored in the buffer unit 10 are counted. In the selector 23, themaximum value is selected by comparing the values of counters 211 to 21nwith each other, and if there are plural maximum values, the one thathas not been selected for a longer time is to be selected.

The second controller 30 performs a function of selecting a bufferhaving the count value of a the number of selecting each buffer BF1 toBFn which corresponds to an allowance r which is not selected at itsmaximum in the buffers BF1 to BFn. The second controller 30 is comprisedof counters CNTr1 to CNTm for counting the read clock CLKrd1 to CLKrdnused upon really reading the cell from each buffer BF1 to BFn, r-valuecomparators 321 to 32 n for comparing the counter values outputted fromeach counter 311 to 31 n to reach the allowance value r with each other,and a selector 33 for sequentially selecting one from the values whichreach the allowance r upon there exist a plurality of values reading theallowance r.

The third controller 40 selects one among the buffers selected in thefirst and second controllers 20 and 30, and controls to read the cellfrom the selected buffer. The third controller 40 is comprised of aselector 41 for receiving the outputs of the first and secondcontrollers 20 and 30, and selecting one having a priority, and amultiplexer 42 for outputting the cell output of the buffercorresponding to the buffers BF1 to BFn according to the output of theselector 41.

Thus, in the multiplexing device configured as shown in FIG. 3, thebuffer unit 10 stores the cells received via the input port in eachcorresponding buffer. The first controller 20 selects the buffer wherethe most cells are stored by counting the write clock CLKwr supplied toeach buffer BF1 to BFn, and the second controller 30 selects the bufferhaving an allowance r which is not most selected by counting the readclock CLKrd supplied to each buffer BF1 to BFn. Thus, the thirdcontroller 40 multiplexes and outputs the cell stored in the bufferhaving a priority by the buffer unit 10 according to the selectionsignal outputted from the first and second controllers 20 and 30.

The operation of the multiplexing device according to the presentinvention will be described 4 a referring to FIG. 3. When a plurality oftraffic inputs are to be multiplexed in a place where the trafficcharacteristic of service to be provided as depicted in an asynchronoustransmission mode network, the above buffers BF1 to BFn should bemanaged efficiently, i.e., economically. For this purpose, themultiplexing device according to the present invention counts the numberof cells stored in each buffer BF1 to BFn and compares the countedresults with each other to read out the cell from the buffer where themost number of the cells are stored. Thus, the overflow can be preventedfor being generated in a particular buffer or the efficiency in usingthe buffer can be prevented from being generated. The reason of countingthe number of the cells in each buffer is to read out the cell from abuffer which is most probable of generating the buffer overflow byrecognizing a fact that which buffer has most cells.

In the buffer control method according to the present invention, thenumber of cells stored in each buffer is counted and compared to readout a cell by selecting a buffer having the greatest value using thefollowing control algorithm.

First, when the number of the cells stored in the buffers BF1 to BFn iscounted, a basic unit i is to be selected as a cell period of c units(c=1, 2, 3, . . . ) so that the output control speed of the cell can bedynamically adjusted according to the change of the traffic amount. Inthe present invention, the cell period T1 is assumed as 2.76 us.

Second, the buffer having the greatest value is selected by comparingthe number of cells stored in the buffers BF1 to BFn with each other anddetecting the greatest value m. The number of the stored cells withrespect to each buffer BF1 to BFn are indicated by reference numeralsb1, b2, b3, . . . , bn, respectively.

Third, when the buffer storing the greatest number of the cells isselected among the buffers BF1 to BFn, the number of the cells to beread out from the corresponding buffer is selected as j units 0=1, 2, 3,. . . ) to dynamically adjust the cell storing state in a bufferaccording to the change of the traffic amount. That is, the cells areread out as many as the increased value j from the buffer having themost probability of overflow occurrence by increasing the j valuethereto, and otherwise, less cells are to be read out from the buffer bydecreasing the j value.

Fourt, when j<c, the number of cells to be read out from the buffer iswithin the range of C since the comparison is performed by the “i” unitwhich is the cell period of c units. Here, an idle cell is removed froma physical layer so that the cell input from the physical layer to anATM layer becomes 2.76 us or less.

Fifth, in case when there is the same cell count comparison value of thebuffers (b_(x)=b_(y), where x and y are arbitrary integers), a bufferb_(old) which has not been selected for a longest time is selected. Toperform such control, the numbers (b_(sn)=1, 2, 3, . . . ) of selectionto each buffer are counted and the counted value should be comparable.When b_(s1)=b_(s2)=b_(s3)=. . . =b_(sn), the sequence of selection isb_(s1), b_(s2), b_(s3), . . . b_(sn).

Sixth, upon b_(old)=r, the corresponding buffer can be primarilyselected. Here, ‘r’ is set by consideration of the characteristic ofservice quality such as jitter or transmission delay.

Seventh and the last, when m<j, the number of the cell to be read outfrom the corresponding buffer is set as the value “m.”

FIG. 4 is a flowchart for explaining a method for multiplexing cells ofan asynchronous mode according to the present invention. In FIG. 4,b_(n) (wherein n=1, 2, 3. . . ) is denoted as a buffer number; b_(old)is denoted as a buffer which has not been selected for the longest time;and b_(sn) (n=1, 2, 3, . . . ) is denoted as the selection number ofeach buffer. Also, “i” is denoted as the number of cells to be read at atime from the selected buffer; “m” is denoted as the number of cells ofa buffer where the greatest number of cells are stored; and ‘r’ isdenoted as the number of the cell time of the unselected buffer.

Referring to FIG. 4, in step 411, values i and j are set. Herein, i isselected by cell periods of c units (where c=1, 2, 3, . . . ) as a countunit of cells inputted to a buffer and j signifies the number of cellsto be successively read out at a time from the selected buffer. Next, instep 412, the cells inputted to each input buffer are counted. Here, thecell count method is i and the value i is @ proportional to cell unit cmultiplied by an integer (wherein i=1c, 2c, 3c, . . . and 11 C″ iscell). That is, when i =5c, every 5 cells is counted as one upon beinginputted. There is difference in the application of the trafficcharacteristic by selecting an appropriate i value during setting callconsidering the traffic amount of each service.

In step 413, it is checked whether b_(sn) indicating the number ofselection of each buffer is the same as “r” which is a time number thatthe buffer is not selected, and if so, the buffer that b_(sn)=r isselected at step 414 to proceed to step 420. However, if not in step413, steps 416 and 417 are proceeded, in order that the maximum value(m) can be searched by comparing the values (b1, b2, b31 . . . bn) ofcounting cells stored in each buffer with each other at step 415. Here,when the maximum value m is determined to be plural in step 416, thebuffer bold which has not been selected for the longest time is selectedin step 417. After that, steps 418 and 419 are proceeded thereafter, andbuffers are selected in an order of b_(s1), b_(s2), b_(s3), . . . ,b_(sn) in the case of setting plural buffers bold (in b_(sx), x is anarbitrary integer among 1 through n).

When the maximum value m is not plural in step 416, or after a buffer isselected by proceeding step 414 or 419, the buffer count value changedin step 420 and the value b_(old) are adjusted. In step 421, “m” whichis represented as the cell numbers stored in the buffer where the mostcells are stored, is compared with “j” which is the cell number of thecells to be read out at a time from the buffer are compared. At thistime, if m<j, i.e., the number of cells stored in the selected buffer isless than that of the cells to be read out, j is changed into m in step422 and cells are read out from the selected buffer in step 423. Here,the cell number of the cells to be read out is theinteger-multiplication of j and the j can be selected (0=1, 2, 3, . . .), namely, when j=S, cells are read out by 5 units. When m>j, step 423is proceeded. The value j can be set as a different value depending onthe buffers. Since the burstiness of traffic is different from eachother, in case of the traffic of great burstiness, the value j isincreased to thereby read out cells from a buffer as many as that, andon the contrary, in case of traffic of lower-burstiness, the value j isdecreased so that a lesser number of cells is to be read out from thebuffer.

When a service quality parameter is reconsidered during the proceedingof call, the renegotiation is detected in step 424 and the values i andj are updated in step 425. The values i and j are selected when the callis set and can be changed during the call procession. Since thestandardization is made such that the renegotiation to traffic parametercan be available when the call is proceeded, the traffic characteristiccan be changed during the call procession.

As described above, the multiplexing device according to the presentinvention efficiently multiplexes the cell traffic by controlling thecell buffer according to the traffic amount. Thus, the multiplexingdevice according to the present invention can improve a service qualityfactor such as cell damage or cell transmission delay occurring when adesignated buffer having a fixed size is used for each cell input port,as well as provide advantages as all ports share the buffer in terms ofthe efficiency in using the buffer.

The present invention can be applied to a terminal equipment(hereinafter, referred to as TE), a transmission equipment including aNetwork Termination½ (NT-½), a concentrator and a multiplexer, and anexchange system, ie., all equipment relating to a communication network.

Also, the present invention can be designed using a buffer (memory), acounter logic, a selector and other passive devices. These devices arecommercialized components and fast enough to operate at a unit of 2.7 usso that purchase and circuit design can be facilitated.

Therefore, it should be understood that the present invention is notlimited to the particular embodiments disclosed herein as the best modecontemplated for carrying out the present invention, but rather that thepresent invention is not limited to the specific embodiments describedin this specification except as defined in the appended claims.

What is claimed is:
 1. A cell multiplexing device of an asynchronoustransmission mode, comprising: a buffer unit which includes a pluralityof cell buffers corresponding to input ports, for storing cells receivedthrough said input ports; a plurality of counters, each of said pluralof counters respectively corresponding to a corresponding cell buffer,each if said plurality of counters respectively storing a numbercorresponding to cells stored in each of said cell buffers; a unit clockunit determining for each of said plurality of counters a count unit ofcells stored in said plurality of cell buffers; a selector outputtingcontrol signals to select cell buffers in an order of selection ofmaximum values by comparing values of said plurality of counters forcorresponding cell buffers; and a unit receiving said control signals,said unit receiving an output of said buffer unit and multiplexing cellsfrom said buffer unit according to said control signals, said unitprocessing cells respectively stored in said cell buffers to be outputfrom said buffer unit according to said order of selection of maximumvalues where most cells are stored.
 2. A method for multiplexing cellsof an asynchronous transmission mode, comprising the steps of: storingcells received from input ports in a plurality of cell buffers of abuffer unit; counting a number of cells inputted to each of saidplurality of cell buffers by a plurality of counters each respectivelycorresponding to one of said plurality of cell buffers; storingrespectively in each of said plurality of counters a numbercorresponding to cells stored in each of said plurality of cell buffers;determining for each of said plurality of counters a count unit of cellsstored in said plurality of cell buffers; determining an order ofselection of maximum values by comparing cell count values of each ofsaid plurality of cell buffers; selecting a cell buffer having a maximumvalue according to said order of selection of maximum values; receivingan output of said buffer unit and multiplexing and outputting cellsstored in said selected cell buffer from said buffer unit according tosaid order of selection of maximum values; and processing cellsrespectively stored in said plurality of cell buffers to output fromsaid buffer unit cells of each said selected cell buffer according tosaid order of selection of maximum values where most cells are stored.3. The method as claimed in claim 2, further comprised of said selectingstep selects from said plurality of cell buffers according to said orderof selection of maximum values, and cells stored in said plurality ofcell buffers are multiplexed and output according to said order ofselection of maximum values.
 4. The method as claimed in claim 3,further comprised of said selecting step selects from said plurality ofcell buffers a cell buffer that has not been selected for a longer timefrom among cell buffers each corresponding to a same said maximum value.5. The method as claimed in claim 2, further comprised of said selectingstep selects from said plurality of cell buffers a cell buffer that hasnot been selected for a longer time from among cell buffers eachcorresponding to a same said maximum value.
 6. The cell multiplexingdevice as claimed in claim 1, further comprised of one of said pluralityof cell buffers that has not been selected for a longer time from amongcell buffers each corresponding to a same maximum value is selected forsaid processing.
 7. A cell multiplexing device of an asynchronoustransmission mode, comprising: a buffer unit which includes a pluralityof cell buffers corresponding to input ports, for storing cells receivedthrough said input ports; a plurality of counters, each of saidplurality of counters respectively corresponding to a corresponding cellbuffer, a selector outputting control signals to select cell buffers inan order of selection determined by comparing read count values andwrite count values of said plurality of counters for corresponding cellbuffers; and a unit receiving said control signals, said unit receivingan output of said buffer unit and multiplexing cells from said bufferunit according to said control signals, said unit processing cellsrespectively stored in said cell buffers to be output from said bufferunit according to said order of selection.
 8. The cell multiplexingdevice as claimed in claim 7, further comprised of one of said pluralityof cell buffers that has not been selected for a longer time from amongcell buffers each corresponding to a same value in said order ofselection is selected for said processing.
 9. A method for multiplexingcells of an asynchronous transmission mode, comprising the steps of:storing cells received from input ports in a plurality of cell buffersof a buffer unit; counting a number of cells input to each of saidplurality of cell buffers and output from each of said plurality of cellbuffers by a plurality of counters; storing write count values eachrespectively corresponding to cells stored in each of said plurality ofcell buffers; storing read count values each respectively correspondingto cells output from each of said plurality of cell buffers; determiningan order of selection of cell buffers by comparing said read countvalues and said write count values for each of said plurality of cellbuffers; selecting a cell buffer of said plurality of cell buffersaccording to said order of selection; receiving an output of said bufferunit and multiplexing and outputting cells stored in said selected cellbuffer from said buffer unit according to said order of selection; andprocessing cells respectively stored in said plurality of cell buffersto output from said buffer unit cells of each said selected cell bufferaccording to said order of selection.
 10. The method as claimed in claim9, further comprised of said selecting step selects from said pluralityof cell buffers a cell buffer that has not been selected for a longertime from among cell buffers each corresponding to a same value in saidorder of selection.